U.S. patent number 4,181,757 [Application Number 05/870,232] was granted by the patent office on 1980-01-01 for process for surface coating gold alloys onto a metallic substrate to enhance corrosion protection.
Invention is credited to William V. Youdelis.
United States Patent |
4,181,757 |
Youdelis |
January 1, 1980 |
Process for surface coating gold alloys onto a metallic substrate
to enhance corrosion protection
Abstract
Processes and compositions are disclosed for surface coating of
metals and alloys of low nobility with gold alloys to provide
improved corrosion resistance and surface hardness to the
underlying metals.
Inventors: |
Youdelis; William V. (Windsor,
Ontario, N9E 1G6, CA) |
Family
ID: |
4109873 |
Appl.
No.: |
05/870,232 |
Filed: |
January 17, 1978 |
Foreign Application Priority Data
Current U.S.
Class: |
427/229; 420/507;
427/383.7; 419/9; 106/1.18; 427/310; 427/376.8; 433/218 |
Current CPC
Class: |
A61K
6/844 (20200101); C23C 24/106 (20130101) |
Current International
Class: |
A61K
6/04 (20060101); A61K 6/02 (20060101); C23C
24/00 (20060101); C23C 24/10 (20060101); B05D
003/02 () |
Field of
Search: |
;427/2,383C,383D,310,229,191 ;428/672 ;75/165 ;106/1.18 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Smith; John D.
Attorney, Agent or Firm: Beveridge, DeGrandi, Kline &
Lunsford
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A process to provide an improved surface corrosion resistance
and surface hardness to a substrate composed of a metal less noble
than gold, which process comprises:
(a) surface coating the less noble metal substrate with a slurry
composed of (i) a gold alloy powder, said gold alloy having a
melting point less than 600.degree. C., (ii) an essentially
inorganic oxide-dissolving flux which becomes fully active in a
temperature range of about 550.degree. C. to 600.degree. C. and
(iii) an organic liquid;
(b) firing the slurry coating to a temperature of about 540.degree.
C. and above to melt and spread the gold alloy over the surface of
the less noble underlying metal substrate.
2. The process of claim 1 wherein said gold alloy powder is
composed of at least one of powdered binary gold alloys selected
from the group consisting of gold-germanium, gold-silicon and
gold-tin.
3. The process of claim 1 wherein said gold alloy consists
essentially of from about 8 to 16% by weight germanium and the
remainder gold.
4. The process of claim 1 wherein the gold alloy consists
essentially of from about 3 to 9% by weight silicon and the
remainder gold.
5. The process of claim 1 wherein the gold alloy consists
essentially of from about 15 to 25% by weight tin and the remainder
gold.
6. The process of claim 1 wherein the gold alloy powder and the
oxide-dissolving flux are combined to form a powder composite
containing up to 5% by weight of the oxide-dissolving flux and the
remainder being gold alloy powder, the gold alloy being composed of
at least one of the powdered binary gold alloys selected from the
group consisting of gold-germanium, gold-silicon, and gold-tin.
7. The process of claim 1 wherein the organic liquid contains at
least one of the constituents selected from the group consisting of
ethylene glycol, cyclohexanol and ethyl or methyl alcohol.
8. The process of claim 1 wherein the organic liquid consists
essentially of equal parts by volume of ethylene glycol,
cyclohexanol and ethyl or methyl alcohol.
9. The process of claim 1 wherein the oxide-dissolving flux and
organic liquid are combined to form an emulsion, the emulsion
containing up to 15% by weight of the oxide-dissolving flux and the
organic liquid containing at least one of the group consisting of
ethylene glycol, cyclohexanol and ethyl or methyl alcohol.
10. The process of claim 1 wherein the slurry contains 5% maximum
by weight of the oxide-dissolving flux, 15% maximum by weight of an
organic liquid containing at least one of the constituents selected
from the group consisting of ethylene glycol, cyclohexanol and
ethyl or methyl alcohol and the remainder comprising at least one
of a powdered binary gold alloy selected from the group consisting
of gold-germanium, gold-silicon and gold-tin.
11. The process of claim 1 wherein the inorganic oxide-dissolving
flux is composed principally of borates, carbonates, fluorides and
chlorides of the alkali metals.
Description
This invention relates to processes and compositions for surface
coating of metals and alloys of low nobility with
low-melting-temperature gold alloys to impart an improved corrosion
and tarnish resistance to the underlying or parent metal which
approaches that of pure gold, as well as providing a surface
hardness that is equal to or surpasses that of many heat-treatable
gold alloys.
Alloys which are used for dental prostheses such as inlays, crowns,
bridges and partial dentures, must provide several essential
properties which include good casting ability, ease of fabricating,
corrosion resistance and hardness. These properties are developed
best in alloys of high precious metal content; however, even high
precious metal content alloys fail to satisfactorily meet all of
the clinical requirements in all applications, in particular
hardness, which decreases as the gold content is increased to meet
the required corrosion resistance. There is thus a need,
particularly for dental work, to provide materials with good
casting and working properties while still retaining good hardness
and corrosion resistance.
It is an object of the present invention to provide a process and
materials or compositions therefor, for coating metal surfaces with
low-melting-temperature gold alloys, which are differentiated from
electroplating with pure gold, to impart high corrosion resistance
and surface hardness to the underlying or parent metal which has a
lower corrosion resistance and hardness. The underlying or parent
metal may be chosen to take advantage of certain specific
characteristic properties, e.g. ability to be easily cast and the
ability to be easily fabricated as well as low cost, with the
required corrosion resistance being provided by a thin surface
layer of gold alloy which is integrally bonded to the underlying or
parent material.
By the present teachings, a process has been developed for
integrally bonding a layer of low-melting-temperature gold alloy of
controlled thickness onto the surface of less noble metals that is
particularly suited to alloys such as silverbase or copper-base
alloys. By the present process surface details of the underlying
metal are accurately reproduced provided that the gold alloy layer
deposited in one application does not exceed about 100 to 150
microns in thickness. The surface corrosion resistance obtained is
equivalent to that of the gold alloy employed for the coating;
however, the corrosion resistance is noted to decrease with depth
into the coating due to increasing alloying of the surface layer
with the underlying or parent metal.
The preferred process for coating the underlying or parent metal
with the protective gold alloy may be as follows:
(a) coating as by brushing, painting, spraying, etc., the surface
of the parent metal with a slurry composed of a
low-melting-temperature gold alloy powder, an essentially inorganic
oxide-dissolving flux which becomes fully active at above about
550.degree. C. and an organic liquid; and
(b) firing the slurry coating to a temperature at about 540.degree.
C. and above to melt and spread the gold alloy contained in the
slurry over the parent metal surface. The heat may be applied
employing an open gas flame i.e. a Bunsen burner, propane torch
etc. or may be applied by an electric or induction furnace.
The slurry which is coated onto the parent metal is composed
essentially of three major constituents: a low-melting-temperature
gold alloy powder, an oxide-dissolving flux powder or emulsion and
an organic liquid which is the vehicle for spreading the gold alloy
and flux particles over the parent metal surface. The gold alloy
should be in particular form and preferably pass through a 400 mesh
screen (or finer) to facilitate the application of a slurry layer
of uniform thickness.
The preferred low-melting temperature gold alloy for the coating
material is a eutectic or near eutectic gold-germanium alloy
containing about 88% gold and about 12% germanium by weight. Other
gold alloys which may be employed are the eutectic alloys of
gold-silicon and gold-tin, containing respectively about 94% gold
and about 6% silicon by weight and about 80% gold and about 20% tin
by weight. The melting temperatures of these eutectic gold alloys
are about 356.degree. C. for the gold-germanium, approximately
370.degree. C. for the gold-silicon and about 280.degree. C. for
the gold-tin alloy. A combination of any two or three of these
eutectic alloys may also be employed as the coating material.
The slurry also contains an oxide-dissolving flux and the role of
the flux in the slurry is to dissolve surface oxides and increase
the wetting and spreading action of the liquefied gold alloy. For
applying coatings to silver-base and copper-base alloys several
commercially available brazing fluxes are suitable for this
purpose, e.g. Handy Flux, manufactured by Handy and Harman, the
principal oxide-dissolving ingredients in these fluxes being
borates, carbonates, fluorides and chlorides of the alkali metals
sodium and potassium and the fluxes become fully active in a
temperature range of about 550.degree.-600.degree. C.
The flux may be employed in powder form and may be either admixed
with the gold alloy powder or the flux may be incorporated into the
organic liquid vehicle. When added to the gold alloy powder about
2% by weight of flux is sufficient. On the other hand, when the
flux is incorporated into the organic liquid about 15% by weight
would be required. It should be noted that it is important to avoid
adding too much flux in view that bubbling and rising of the slurry
layer may become excessive when fired due to evolution of water of
hydration from the flux constituents.
The organic fluid for the slurry is employed as a vehicle for
spreading the gold powder and flux particles over the surface of
the underlying or parent metal. The vapor pressure of the organic
fluid should be sufficiently high so that the slurry will dry
relatively quickly when applied by brush or spray. The liquid
should also have some water solubility otherwise the flux particles
may tend to agglomerate. High vapor pressure liquids have
correspondingly low boiling points, and there are several organic
liquids which would have boiling points below that of water
(100.degree. C. ) and are also water soluble to some degree. The
preferred organic liquid vehicle is a mixture of three liquid
organic constituents and is composed of about 1 part of ethylene
glycol, about one part cyclohexanol and about one part ethyl or
methyl alcohol by volume.
Thus, in accordance with the present teachings, in addition to the
process of coating there is also provided a powder composite
composed of about 1 to 5% by weight of a flux powder containing
oxide-dissolving constituents and the remaininder a gold alloy
powder, the gold alloy powder being about 88% gold and about 12%
germanium by weight. In a further aspect of the invention there is
also provided an organic liquid to function as a vehicle for
applying gold alloy and flux powder constituents onto the
underlying or parent metal surface, the organic liquid being
composed of about 1 part ethylene glycol, about one part of
cyclohexanol, and about 1 part ethyl or methyl alcohol by
volume.
The gold alloy-flux powder composite together with the organic
liquid may be mixed in such proportions as to provide a slurry of
desired fluidity for brushing or spraying onto the underlying or
parent metal surface.
After the application of the slurry to the underlying or parent
metal surface, heat is then applied to melt the slurry coating and
to spread the gold alloy over the surface of the metal. This
heating may be accomplished by an open gas flame or by a furnace.
No special precautions are necessary against oxidation during
firing although a reducing or inert atmosphere may be preferred. If
the slurry coated underlying or parent metal is heated over an open
gas flame, such as a Bunsen burner or propane torch, the heat
should be applied slowly and uniformly by continually moving the
flame over the whole surface to avoid localized overheating,
otherwise lifting or spalling of the slurry coating may result due
to a too rapid vaporization and effusion of the organic
constituents. For the fluxes and gold alloy compositions of the
present concept, a temperature of about 600.degree. C. is adequate
to melt and spread the alloy and should not be exceeded. With
experience, an operator can readily estimate when the required
temperature is attained by the "flash" or rapid spreading of the
gold alloy over the surface. After the flash has occurred the flame
should then be removed to avoid excessive heating or alloying of
the gold alloy with the underlying or parent metal.
The slurry coating which has been applied to the underlying or
parent metal may also be fired in an electric or induction furnace
under more precisely controlled conditions. Again, heat must not be
applied too rapidly in order to avoid excessive lifting or
localized spalling of the slurry coating. When an electric
resistance furnace is used a recommended procedure which may be
followed is to place the slurry-coated underlying or parent metal
into the furnace chamber, which is at a temperature of about
540.degree. C. but not exceeding 560.degree. C., maintain the
temperature for 3 to 5 minutes and then raise the temperature to
about 600.degree. C. which would require up to about 5 minutes. The
temperature of 600.degree. C. is then held for an additional 3 to 5
minutes and the composite is then removed and quenched in water.
The total elapsed furnace time should run about 10 to 15 minutes.
During the initial 3 to 5 minute period at 540.degree. to
560.degree. C. the flux in the slurry coating slowly liquifies with
a minimum of rising and bubbling and removes any surface oxides as
it spreads over the surface of the underlying or parent metal. The
gold alloy powder also liquifies and spreads over the surface at
the same time. The latter 3 to 5 minute period in the furnace held
at a temperature of about 600.degree. C. ensures completion of the
spreading of the liquid gold alloy by surface tension forces.
The thickness of the gold alloy coatings may readily be controlled
by varying the thickness of the slurry coating with a continuous
layer of gold alloy of from several microns up to 150 microns in
thickness being deposited in a single application. If a thickness
exceeding 100 microns is desired it is recommended that the coating
be applied in two or several stages. Any single application should
not be so thick that gravity forces exceed surface tension forces
and cause flow of the liquid alloy to lower areas of the structure
being coated.
For purposes of illustration and not limitation, the following are
examples of metals and alloys which were readily and effectively
surface coated utilizing the procedures and constituents disclosed
above.
__________________________________________________________________________
Underlying (Parent) Metal Surface Coating Example Hardness Tarnish
Hardness Tarnish No. Alloy Vickers (1%NaS) Alloy Vickers (1%NaS)
__________________________________________________________________________
1. Ag-Cu-Ge (71.1-27.6-1.3) 95 brown 88Au-12Ge 190-280 nil 2.
Ag-Cu-Ge (71.1-27.6-1.3) " brown 94Au-6Si 150-280 nil 3. Ag-Cu-Ge
(71.1-27.6-1.3) " brown 80Au-20Sn 150-250 nil 4. Brass (70Cu-30Zn)
75 orange 88Au-12Ge 190-280 nil 5. Brass (70Cu-30Zn) " orange
94Au-6Si 150-280 nil 6. Brass (70Cu-30Zn) " orange 80Au-20Sn
150-250 nil 7. Copper 60 black 88Au-12Ge 190-280 nil 8. Mild Steel
130 nil 88Au-12Ge 190-280 nil
__________________________________________________________________________
To further illustrate the present invention reference may be had to
the drawings wherein:
FIG. 1 is a photomicrograph of a dental crown, magnification X8,
and
FIG. 2 is a photomicrograph of the gold layer of the wall of the
crown of FIG. 1, having a magnification of X200.
With particular attention to FIG. 1, the dental crown as shown has
been coated with the eutectic gold-germanium alloy containing 88%
gold and 12% germanium by weight. The crown material is a
silver-copper-germanium alloy such as that described in U.S. Pat.
application Ser. No. 809,764, and would correspond to Example 1 of
the present application. The polished cross-section of the crown
has been exposed to a 1% sodium sulphide solution which has
tarnished or etched the parent metal but not the gold alloy layer.
The gold alloy coating is about 150 microns in thickness and is
clearly distinguishable from the parent or underlying material.
FIG. 2 is a higher magnification of the gold layer of the wall of
the crown of FIG. 1. The surface region of the gold layer shows the
characteristic microstructure of eutectic gold-germanium alloy. The
diffusion zone, zone B, where integral bonding is developed is
about 75 microns in thickness and has the Vickers hardness of 160.
The surface region of the gold alloy layer, zone A, has a thickness
of about 70 microns and a Vickers hardness of 280. The parent metal
has a Vickers hardness of 95.
By the present concept, one is able to employ an underlying or
parent metal which may be chosen with respect to certain
characteristic properties such as ease of casting and fabricating
as well as being a low cost material. The thin surface layer of
gold alloy which is integrally bonded to the underlying metal
provides the hardness and corrosion resistance necessary for dental
prostheses.
While the principals of the invention have been made clear with
particular reference to certain preferred embodiments, it will be
understood that variations and modifications can be effected by one
skilled in the art within the spirit and scope of the
invention.
* * * * *